This invention relates generally to tankers for carriage of liquids such as oil and, more particularly, to a system for control of oil leakage from a damaged tanker. In a more specific sense, the invention relates to improvements to the oil leakage control system described in applicant's U.S. Pat. No. 4,964,437 dated Oct. 23, 1990.
Briefly, the system described in the patent, the disclosure of which is hereby incorporated herein by reference, minimizes the outflow of oil from a damaged cargo tank of an oil tanker of the "segregated ballast" type by transferring oil out of the upper part of the damaged cargo tank and at the same time, keeping to a minimum the reduction of draft at the damaged area. This is accomplished by valves and piping installed through selected bulkheads for connecting each cargo tank to one or more ballast tanks, one or more of the valves being opened upon occurrence of damage sufficient to cause cargo tank leakage, allowing oil to flow, by gravity, from the upper part of the damaged cargo tank or tanks to one or more ballast tanks which, if the tanker were loaded, would be empty. Each connection between a cargo tank and a ballast tank includes two valves, either butterfly or sluice valves, connected in tandem, as required by regulations to prevent oil contamination of ballast as any occur from valve leakage in normal operation. The valves typically have a diameter in the range from three to five feet, the choice being a tradeoff between the cost of the valves and the rate at which it is desired to transfer oil from the damaged cargo tank to the ballast tank(s), and each is equipped with an actuator for opening and closing the valve. Considering that a typical tanker may require at least ten such pairs of valves to effectively control oil leakage by this method, the installation obviously is expensive.
A less expensive and more effective valve structure for use in the system is described in applicant's co-pending application Ser. No. 07/510,932 filed Apr. 19, 1990, entitled "Valve Structure". The disclosed valve structure includes a butterfly valve bolted to one end of a short section of pipe which is welded to the bulkhead separating a cargo tank from a ballast tank in combination with a rupturable disk fitted within the pipe section between the rotatable disk of the butterfly valve and the bulkhead and capable of withstanding the maximum pressure of either a full ballast tank or a full cargo tank. A cutter is secured to the rotatable disk of the butterfly valve which, when the butterfly valve is opened in response to detected leakage from an associated cargo tank, slices the rupturable disk sufficiently to assure its collapse and failure by oil flowing through the open butterfly valve. Although less expensive than two butterfly or sluice valves in tandem, this structure is subject to corrosion and not readily amenable to testing.
It is a primary object of the present invention to provide an improved system for minimizing the outflow of oil from a damaged tanker of the "segregated ballast" type.
Another object of the invention is to provide a valve arrangement for initiating oil flow from a damaged cargo tank to a ballast tank which is more effective for the purpose, and less expensive, than paired butterfly valves or the butterfly valve/rupturable disk combination.
Another object is to provide a valve structure which minimizes the possibility of leakage between a cargo tank and a ballast tank without relying on valve seals.